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    SubjectRe: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb
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    On Sat, 2009-09-05 at 14:26 +0100, Jamie Lokier wrote:
    > Richard Kennedy wrote:
    > > I've been testing this & it works pretty well here, but setting
    > > max_writeback_mb to 128 seems much too large for normal desktop machines.
    > >
    > > Because it is so large the background writes don't stop when they get
    > > down to the background threshold, but just keep on writing.
    > > background_threshold on my machine is only about 300Mb so it can
    > > undershoot by quite a bit. This could impact random write workloads
    > > significantly.
    >
    > If that's true, would it be even worse for embedded devices with, say,
    > just 32MB RAM? It sounds like writeback undershoot might be rather
    > extreme in that case.

    Well, on a machine that small I don't think it will be any worse. The
    current code tries to write 1024 pages so its undershoot will be about
    100% anyway.

    > Also on this topic, should max_writeback be smaller for slow disks? I
    > have a small device here with a hard disk that can only be written at
    > 2-10MB/s due to limitations of the built-in IDE controller.
    >
    > I know that's unusual, but it shows there is quite a wide range of
    > speeds at which disks can be written, even just counting hard disks.
    >
    > -- Jamie

    I'm not sure about that, it will depend on how the background threshold
    issue gets fixed.

    regards
    Richard




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